The present invention relates to the manufacture of alkali metal hydroxides, commonly known as caustic. Alkali metal hydroxides are typically from the group which includes sodium, lithium, cesium, potassium and rubidium. Caustic is made using different types of electrolytic cells, including membrane and diaphragm-type cells and then further treating the caustic to concentrate the solution. The present invention is directed to the method of further treating sodium hydroxide caustic which was created using a diaphragm electrolytic chlorine cell, but it is contemplated that this method of manufacture can be utilized in other alkali metal hydroxide processes to reduce the plating of metal on production equipment.
A need has long existed in the caustic industry for a technique to remove metals, such as nickel and/or iron from build up on the insides of manufacturing vessels and piping which occurs during the caustic manufacturing process without adding additional chemicals to the process. A need has existed for an environmentally friendly method to reduce metal plating buildup which lowers the cost to manufacture caustic, lowers the turn around time for the manufacturing facilities and can be usable in any alkali metal hydroxide manufacturing process.
Alkali metal hydroxide solutions have been prepared by the electrolysis of alkali metal salt solutions in electrolytic cells. Diaphragm cells are known for producing alkali metal hydroxide solutions electrolytically. In this type of cell, a porous cathode with an overlying porous diaphragm is used to separate or to serve as a barrier between the catholyte and anolyte compartments of the cell. After separation of the components, the caustic solution is then further treated to increase the concentration of caustic in the solution. An objectionable feature of known methods for handling this process to increase caustic concentration is that the process can require the addition of sodium borohydride to the caustic solution to minimize the reaction of hot caustic with the first device for driving off water, which is commonly known as the "first effect evaporator". The first effect evaporator is usually nickel or coated nickel and the caustic reacts with the nickel causing the build up of hydrated nickel oxide on the interiors of the remaining processing equipment. The use of the sodium borohydride has produced a significant build up of nickel oxides as well as free nickel in the manufacturing process on the vessels, pipes and valves used in the process. The build up on the processing equipment results in increased maintenance and decreased operating efficiency. This invention overcomes this buildup.
The present invention is directed at a method for reducing the amount of free nickel and nickel oxides, free iron and ferrous oxides in the chlor-alkali systems, particularly for the diaphragm types of cells without the need for additional chemicals, additional sodium borohydride, or other contaminants.
In order to overcome the disadvantages of the prior art the present invention proposes the use of magnets in situ in the manufacturing process to overcome these problems with the nickel and the iron.